Harris, G. 1993. Nitrogen cycling in animal-, legume- and fertilizer-based cropping systems. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

Managing nitrogen (N) for maximum crop use efficiency and minimum environmental impact in agricultural systems that use organic versus fertilizer N inputs requires more information on N cycling in these systems. The overall objective of this research was to compare N cycling in animal-, legume-, and fertilizer-based cropping systems with an emphasis on evaluating each system for its potential to lose N to the environment. Specific objectives were 1) to trace the fate of red clover (trifolium pratense L.) and ammonium sulphate 15N into crops plus soil, and calculate loss by subtraction, 2) measure the soil N supplying capacity using long-term laboratory incubations, 3) measure NO3-N leaching using lysimeters, and 4) measure the effect of mineralization-immobilization turnover (MIT) on recovery of legume and fertilizer 15N by corn using a modified 15N method.

15N-labeled red clover and ammonium sulphate were applied to microplots in the long-term Rodale farming systems trial located in east central Pennsylvania in 1987 and 1988 and traced into first-year corn (Zea mays L.), second-year barley (Hordeum distichum L.) and soil after each crop. Over a 2-y period, more fertilizer N was contributed to crops (40 vs. 17% of input), more legume N was contributed to soil (47 vs. 17% of input) and a similar amount of N from both sources was lost from the cropping systems (39% of input).

Long-term (200 + d), aerobic, incubations of soil sampled from the Rodale farming systems in 1988 showed that the N supplying capacity of the animal-based cropping system was greater than the legume- and fertilizer-based systems which were similar. A double exponential model was used to estimate the size of the active fraction of soil organic matter N, which was greater in the animal-than legume-based system, which in turn was greater than in the fertilizer-based system.

Nitrate leaching from the animal-, legume-, and fertilizer-based cropping systems at Rodale was measured over a 2-y period starting in Fall 1990 using 0.76 m dia. by 1.0 m deep intact, natural drainage lysimeters. Most NO3-N leached was derived from soil organic matter and not from the applied legume and fertilizer N sources as determined using 15N.

15N-labeled fertilizer and legume plant material were applied to unlabeled soil, and vice versa, infield microplots at two locations in Michigan in 1991. It was determined that MIT had not effect on recovery of alfalfa (Medicago sativa L.), red clover, hairy vetch (Vicia villosa Roth.), or ammonium sulphate 15N by corn.

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